You would need to downgrade your system in order to upgrade to 3.55 CFW. If you can't downgrade it yourself please check out racer's thread over here in the forums great guy to get your system downgraded.Its best that you check if your system is downgrade able to begin with do you have a PHAT console or a SLIM ?? and did you get 3.65 out of the box or did you upgrade to 3.65 OFW.

Step 1: Download the MinVerChk.rar (from the link provided above) and extract the .pup file using WinRar.
Step 2: Put the PS3UPDAT.PUP onto the USB Flash Drive in the PS3/UPDATE/ folder.
Step 3: Power on your PS3 and navigate to the "System Update" option in the settings
Step 4: Choose "Update Via Media Storage"
Step 5: Let the PS3 search for the update, once it's found it it will ask you if you'd like to run it. Hit "OK"
Step 6: It should only take a few seconds, a message will pop up telling you what the lowest version firmware your PS3 can use!
Step 5: Congratulations! You're done!